On Sept. 11, 2001, two men arrived at the ticket counter late for American Airlines Flight 77 out of Dulles International Airport. This was before the days of the Transportation Security Administration, when airport security was quite different from what it is today. At the time, the man working at the counter, Vaughn Allex, followed procedure and checked them through.

Those two men were among the five hijackers who crashed that flight into the Pentagon — killing 189 people, including themselves.

"I didn't know what I had done," Allex recalls, on a recent visit with StoryCorps, in Potomac Falls, Va. He didn't find out until the next day what had happened. "I came to work and people wouldn't look at me in the eye." Officials handed him the manifest for the flight. "I just stared at it for a second and then I looked up, I go, 'I did it, didn't I?' "

He had checked in a retiree's family on that flight. He had checked in a student group, their parents, their teachers.

"And they were gone. They were just all gone."

Once it became clear what had happened, Allex says people stopped talking to him. He began to think that he was to blame for everything that had happened on Sept. 11. That perhaps he could have changed it, if only he'd done something differently.

And for Vaughn, knowing that many around him were struggling with the greater grief of a lost loved one made attending support groups uncomfortable. "How do I sit in a room with people that are, that are mourning and crying and they're like, 'What's your role in this whole thing?' "

What could he say to them? "Well, I checked in a couple of the hijackers and made sure they got on the flight."

Weeks and even months passed like this, when sometimes even a simple mention of Sept. 11 could trigger a brutal wave of guilt. Once, when a customer told him her husband had been killed on that day, what he misheard instead was, "You killed my husband on Sept. 11."

Allex says he's never been able to fully move past the memory. He says it remains with him always in some form or another. But with time, he has managed to start talking about it.

"I feel like in some ways I've — I really have come out of a shadow over the last 15 years," he says, "and I'm — I'm back in the light now."

Audio produced for Morning Edition by Liyna Anwar.

StoryCorps is a national nonprofit that gives people the chance to interview friends and loved ones about their lives. These conversations are archived at the American Folklife Center at the Library of Congress, allowing participants to leave a legacy for future generations. Learn more, including how to interview someone in your life, at StoryCorps.org.